Evidence-based nutritional interventions that achieve and sustain optimal growth in young children with cystic fibrosis (CF) do not exist, despite an urgent need. Such an intervention could positively change the course of clinical lung disease and enhance survival for these children. The objective of this competing continuation study is to conduct a multi-center, randomized, controlled clinical trial comparing a behavioral plus nutrition intervention condition to an attention control intervention condition. All subjects will receive nutritional care consistent with the 2001 CF Consensus Conference guidelines for pediatric nutrition. The specific aims are to: 1. determine the impact of the behavioral intervention on energy intake and weight gain;2. examine the durability of the behavioral intervention's impact on growth (weight and height) one year following treatment', and 3. explore the relation between physical activity and growth. The central hypothesis is that behavioral intervention will lead to better growth as measured by change in weight and height for age z scores. From three accredited CF Centers in Ohio (Cincinnati, Columbus, Cleveland), 100 preschoolers with CF and pancreatic insufficiency age 2 to 6 years will be randomized to one of the two conditions. The two groups will be stratified so that they are similar at the initiation of treatment on weight for age z score. Other critical variables such as history of Pseudomonas aeruginosa infection and gender will be used as covariates in the statistical analysis plan. Outcome data (energy intake measured by 7-day diet record, weight, height) will be obtained at baseline, post-treatment (6 months), and after a 12-month follow-up (18 months post baseline). Secondary measures will include body mass index, body composition measured by DXA and skinfolds, and growth velocity. Behavioral treatment will maximize adherence to a high energy diet and enzyme replacement therapy, and motivate children to increase their energy intake. It involves 7 weekly sessions followed by 4 monthly sessions. The attention condition controls for time of contact and number of assessments conducted. This study advances the investigation of early nutritional interventions for young children with CF and directly addresses the need for controlled, longitudinal assessment of behavioral intervention on growth. The long-range goal is to change the standard of nutritional care for young children with CF because behavioral intervention leads to optimal growth and ultimately improves lung health and survival.